1. Field of the Invention
The present invention relates to an image pickup circuit, a CMOS sensor, and an image pickup device, and particularly to an image pickup circuit, a CMOS sensor, and an image pickup device that can reduce the noise of an image.
2. Description of the Related Art
In the past, a CMOS (Complementary Metal Oxide Semiconductor) sensor as a solid-state image pickup element has advantages such as lower power consumption, higher speed and the like over a CCD (Charge Coupled Device), for example, and has recently been widely incorporated into a portable telephone, a compact digital camera, a high-grade single-lens reflex camera, a camcorder, a monitoring camera, a guidance system and the like.
In addition, a high-performance sensor that outputs a high-quality image has recently been developed in which sensor functional circuit blocks such as an image processing circuit and the like are formed on a chip together with a CMOS sensor.
For example, Japanese Patent No. 3734717 or Japanese Patent No. 3710361 (referred to as Patent Document 1 or 2 hereinafter) discloses techniques in which using a CDS (Correlated Double Sampling) circuit to process an image signal in a CMOS sensor, a received light signal from a photodiode within a pixel is passed through an analog CDS circuit disposed in each pixel column to thereby remove noise included in the pixel signal, and thereafter A/D conversion is performed.
However, in a case of thus using the CDS circuits, there is for example a problem of occurrence of noise in a stripe-shaped fixed pattern due to variations of the CDS circuits in each pixel column, a problem of an increase in circuit area because it is necessary to provide a capacitive element for retaining a signal value after CDS processing, or a problem of susceptibility to switching noise or the like because rapid horizontal scanning of an analog signal is performed by a shift register.
Accordingly, for example, Japanese Patent Laid-Open No. 2005-328135 (referred to as Patent Document 3 hereinafter) proposes a solution to these problems by a parallel column A/D (Analog/Digital) conversion system (hereinafter referred to as a column AD system).
In the column AD system, an A/D converter is placed in each pixel column, and analog signals of respective pixels in selected columns are collectively output to respective vertical signal lines and are then directly subjected to A/D conversion. Therefore, the problems occurring when the CDS circuit as described above is used are solved, and high-precision noise removal can be performed.
Further, in the column AD system, because of parallel processing in each row in a horizontal direction of an image, scanning in the horizontal direction does not have to be driven at a high frequency, and A/D conversion can be driven at a low frequency in a vertical direction. The column AD system therefore has another advantage of being able to separate a noise component occurring in a high-frequency band from a signal component easily.
A configuration of a CMOS sensor employing the column AD system will be described in the following with reference to FIG. 1.
In FIG. 1, a CMOS sensor 11 includes an FD (Floating Diffusion) 12, a transistor 13, a current source 14, a reference voltage circuit 15, a resistance 16, N comparators 171 to 17N, and N counters 181 to 18N.
Incidentally, FIG. 1 shows one of pixels forming a pixel array in which the plurality of pixels detecting light are arranged in the form of a lattice, and does not show the other pixels. Of constituent elements of the one pixel, only the FD 12 and the transistor 13 for detecting a pixel signal are shown, while transistors necessary to read the pixel signal such as a transfer transistor, a reset transistor, a selection transistor and the like and a photodiode are not shown.
As shown in FIG. 1, one terminal of the FD 12 is grounded, and another terminal of the FD 12 is connected to the gate of the transistor 13. The source of the transistor 13 is connected to a power supply voltage VDD for driving. The drain of the transistor 13 is grounded via the current source 14, and is connected to one input terminal of the comparator 171.
The output terminal of the reference voltage circuit 15 is connected to the power supply voltage VDD for driving via the resistance 16, and is connected to another input terminal of the comparator 171. The output terminal of the comparator 171 is connected to the counter 181. As with the comparator 171, the comparators 172 to 17N have one input terminal connected to the drain of a transistor of a pixel not shown in the figure, have another input terminal connected to the output terminal of the reference voltage circuit 15, and have an output terminal connected to the counters 182 to 18N, respectively.
A charge corresponding to an amount of light received by the photodiode not shown in the figure is transferred to the FD 12 to be accumulated in the FD 12. The transistor 13 amplifies the charge accumulated in the FD 12 and then supplies a pixel signal P to one input terminal of the comparator 171. The other input terminal of the comparator 171 is supplied with a ramp signal R output from the reference voltage circuit 15. Then, the comparator 171 outputs a comparison signal indicating a result of comparing the pixel signal P and the ramp signal R with each other to the counter 181. The counter 181 counts a predetermined clock signal according to the comparison signal, and then outputs the count value as pixel data.
In the thus formed CMOS sensor 11, the FD 12 connected to the gate of the transistor 13 has a parasitic capacitance with GND, and the reference potential of the pixel signal P is a GND level, whereas the reference potential of the ramp signal R is the level of the power supply voltage VDD. Thus, for example, when a noise occurs in the power supply voltage VDD, the noise is superimposed on the ramp signal R, and effect of the noise appears in the result of comparing the pixel signal P and the ramp signal R with each other.
FIG. 2 shows an example of laterally drawn noise caused by such noise and occurring in an image. The laterally drawn noise appears as a randomly changing noise.